Cathode ray tubes such as are employed, for example, in oscilloscopes, are easily damaged by excessive beam intensity during periods of time when the electron beam is moving slowly or not at all. If, for example, the operator has adjusted the beam intensity for viewing a relatively fast deflection signal and that signal is interrupted or removed, the resultant high intensity of the electron beam may burn the CRT phosphor and warp or melt the CRT expansion mesh.
One known CRT protection scheme switches a large series impedance into the cathode circuit to reduce beam intensity upon detection of a harmful condition. A disadvantage of this prior art method is that the reaction time of the sensing circuit is a direct function of the magnitude of the x-y deflection at the point of interruption. In the case of a large deflection signal the resulting large reaction time may allow damage to the CRT. Another problem associated with this protection method is the unreliability of high voltage switches which are required to switch the series impedance. Failure of such a switch may cause major damage to the CRT and associated circuitry.
Other prior art CRT protection circuits sense only the velocity of the electron beam. Since these circuits require a minimum beam velocity irrespective of deflection magnitude before protection begins, repetitive signals of low deflection magnitude would not trigger the protection circuit but may nevertheless damage the CRT. In addition, all of these prior art circuits render the intensity control totally inoperative during the limiting period.
Accordingly, it is an object of the present invention to provide a CRT protection circuit having a constant reaction time, as measured from the point at which the deflection signal is interrupted or removed until beam intensity limiting begins.
It is a further object of the present invention to provide a CRT protection circuit in which the operator has control of the beam intensity at all times, including that portion of time during which limiting occurs.
It is yet a further object of the present invention to provide a CRT protection circuit which allows the user to have full control of the beam intensity for a specified period of time after the beam begins moving subsequent to being in a fixed position at minimum intensity.
These objects are accomplished in accordance with the preferred embodiment of this invention by employing a velocity and x-y deflection magnitude sensing circuit having a constant reaction time and a control circuit for clamping the output of the Z-axis (beam intensity) amplifier. The control circuit includes a limiting delay circuit which enables the operator to have full control over beam intensity for a finite time after the beam begins moving subsequent to being stationary with minimum beam intensity.